Double layer cast iron castings

ABSTRACT

CAST IRON CASTINGS HAVING FREE GRAPHITE AGGREGATED CLOSELY IN THE WEAR PORTION OF THE CASTING ARE PREPARED BY INOCULATING MOLTEN CAST IRON WITH SILICON IN AN AMOUNT SUFFICIENT TO PROVIDE IN THE CAST PRODUCT A VALUE OF K OF AT LEAT 0.9, THE VALUE OF K BEING EXPRESSED IN THE FORMULA:   C(%)+0.31SI(%)=K(1.30+2.57X10**3T)   REPRESENTING THE SOLUBILITY OF CARBON AND SILICON IN THE MOLTEN CAST IRON AT TEMPERATURE T* C., CASTING THE MOLTEN METAL INTO A MOLD, AND COOLING THE CAST MOLTEN METAL.

Feb. 23, 1971 sHoGo sAz-:Kl ETAL 3,565,609

4 DOUBLE LAYER CAST IRON CASTINGS Filed Oct. 24, 1967 FIG1 FIGZ F|G3 United States Patent Office 3,565,609 Patented Feb. 23, 1971 3,565,609 DOUBLE LAYER CAST IRON CASTINGS Shogo Saeki, Toshitsugu Ohi, and Minoru Fujioka, Tamano, Japan, assignors to Mitsui Shipbuilding and Engineering Co. Ltd., Tsukiji, Chuoku, Tokyo, Japan, a corporation of Japan Filed Oct. 24, 1967, Ser. No. 677,605 Claims priority, application Japan, Oct. 25, 1966, 41/70,529 Int. Cl. C22c 37/00, 37/10 U.S. Cl. 75-130 2 Claims ABSTRACT OF THE DISCLOSURE Cast iron castings having free graphite aggregated closely in the wear portion of the casting are prepared by inoculating molten cast iron with silicon in an amount suicient to provide in the cast product a value of K of at least 0.9, the value of K being expressed in the formula:

representing the solubility of carbon and silicon in the molten cast iron at temperature t C., casting the molten metal into a mold, and cooling the cast molten metal.

This invention relates to cast iron castings, more particularly to double layer cast iron castings, in a certain portion of which free graphite is presented closely by means of adjusting the molten metal composition and controlling cooling method.

The cast iron is `widely used for the sliding parts of the machine, because the castings has high wear resistance owing to self lubrication of graphite in dry abrading state or to oil in the oil sump formed at the graphite portion in wet abrading state.

The carbon content of the normal cast iron is 3.00% to 4.00%, of which about 0.8% combines with iron with the formation of Fe3C which exists in the structure, and of which remainder is deposited as free graphite and homogeneously dispersed in the flaky or spheroidal form. Therefore, if the free graphite are more increased, the cast iron is more improved in its wear resistance either in dry or Wet state. However, it is diicult to produce more than 4% free graphite in cast iron by any melting method, and excessive graphite is deposited on the surface of the cast iron as Kish graphite.

An object of this invention is to provide a double layer castings in `which free graphite is closely aggregated in a portion of the castings to be used for a Wear surface, for example, the surface portion of the castings or the inner surface portion of the cylindrical castings.

Another object of this invention is to provide a method to deposit graphite closely in a portion of the cast iron during solidifying.

The whole amount of the free graphite in the castings made by this invention is as much as that of the conventional castings, but, in the closely aggregated graphite portion or layer, the amount of free graphite is 2 to 5 times as much as that of the latter in the portion. Therefore, the castings according to this invention is useful for the sliding member or other parts of the machine.

In accordance with this invention, preparing a molten metal of cast iron containing the carbon and silicon in excess of the critical quantity, an inoculant containing silicon is added to the molten metal, and then casting it into the mold. At rst, fine proeutectic graphite crystallizes in the molten metal in great amounts and grows up as the metal gets cold. Since the specific gravity of the graphite is less than that of the molten metal, the graphite rises to the surface of the metal. The graphite in the surface portion gradually increases in amount until the molten metal solidies, thus a closely aggregated graphite layer or Zone is formed in the surface portion of the castings if the graphitization is fast in speed.

The formation of the graphite layer depends on the graphitization speed and the cooling speed of the cast iron. Furthermore, the graphitizing speed depends on the quantity of the carbon and silicon melted in the melt.

This invention provides a relation with respect to solubility of carbon and silicon for formation of the graphite layer as set forth hereinafter.

The solubility of carbon and silicon at temperature t C. is generally expressed by the formula:

where C* and Si* express its percentage in weight at carbon and silicon melting in saturated state.

Therefore, in the case of arbitrary composition, the formula is as follows where K is coefficient and expresses the transiting rate of carbon and silicon from the saturation point.

When K=1 If the value of K calculated from the composition of the molten metal after inoculating is equal to one or slightly less than one, the ne proeutectic graphite crystallizes immediately after casting, and the graphite layer is formed as above described.

If the value of K is much less than one, even though the composition is hypereutectic, the crystallization of the proutectic graphite is delayed. Therefore the graphite can not rise to the surface before solidifcation and the graphite layer can not be formed. The formation of the graphite layer is also iniluenced by cooling speed of the molten metal.

Representing in practical example, if the carbon is 4.0% in weight and the temperature of the molten metal is l400 C., the weight of silicon to satisfy K=l is 2.9% However, in case of using a mold of phenol resin and casting a 30 x 40 x 100 mm.3 test piece, the maximum Weight of silicon is 2.6%. If the weight of silicon is greater than the value, the graphite layer can be formed, and if the weight of silicon is less than the value, the layer can not be formed. When size of the test piece is larger than the above mentioned value, the maximum or critical value of silicon decreases more.

In adding alloying element, if the melting rate of carbon is adjusted according as amount of carbon equivalent to the added element in accordance with the amount of carbon and silicon, the treatment is same as the Fe-C-Si system.

In the case of the spheroidal graphite cast iron, if globularity treatment is previously made before inoculating, proeutectic graphite becomes a spheroidal form. Since the spheroidal graphite has less resistance than that of flake graphite, the rising speed of the spheroidal graphite is higher and a graphite layer of high density is easily obtained.

To form the closely distributed graphite layer in the inner surface portion of the cylindrical castings such as bearing metal or cylinder liner, the centrifugal casting method is employed. In this method, the density and thickness of the graphite layer can be varied by adjusting rotation speed according to the object of castings.

The following examples clearly indicate the method for producing the spheroidal graphite double layer castings of this invention.

The globularity treatment of the molten metal was made by Fe-Si-Mg alloy of 1.5% containing 20% magnesium, and inoculated by Fe-Si of 0.5% (75% silicon) held at a temperature of 1400 C. with the composition of 4.04% carbon and 2.81% silicon, and then casted into the sand mold.

FIG. 1 is a sectional view of a test piece of thickness of 40 mm. The thickness of the closely distributed graphite layer 1 was 3 mm., the amount of the graphite in the layer was 23 volume percent. The normally distributed graphite portion 2 was of 9 to 10 volume percent, thus the density of graphite in the layer 1 was more than twice that of the portion 2.

PIG. 2 is a sectional View taken on line II-II of FIG. 3 which casted by centrifugal casting,

FIG. 3 is a right side view of FIG. 2. The graphite of the layer 1 of this casting was 30 volume percent.

We claim:

1. A method for producing cast iron castings having free graphite aggregated closely in the wear portion of the casting, comprising the steps of inoculating molten cast iron with silicon, the amount of silicon added being suicient to provide in the 4 cast product a value of K of at least about 0.9, in the formula:

where t is temperature in C. of the molten metal when said -rnetal is inoculated with silicon; casting the molten metal into a mold; and cooling the cast molten metal. 2. The method of claim 1 wherein t is about 1400 C. andl wherein K is at least about 0.9 and is not greater than about l.

References Cited UNITED STATES PATENTS 1,746,467 2/1930 Greiner r 75-130X 1,867,732 7/1932 Coyle 75--130 3,033,676 5/1962 Cox 75-130 L. DEWAYNE RUTLEDGE, Primary Examiner 2O I. E. LEGRU, Assistant Examiner U.S. C1. X.R. 

